How can I get a count of all active people in all our lists/smart lists on one report?
This is simply not possible. per se.
But you can use custom columns in people performance reports (see Add Custom Columns to a Person Report - Marketo Docs - Product Documentation ). You will get one column for each of your smart list.
But you can only add a few smart lists to the report, or the report will be very very slow to produce.